1. Field of the Invention
The field of the present invention is signal processing for the output of photodetectors, particularly photodetectors which are used to detect light scattered from a concentration of aerosol particles.
2. Background
Examples of systems that use photodetectors include, but are not limited to, optical character recognition systems, communication systems medical imaging sensors, laser range finders, radiation detectors, smoke detectors, position sensors and proximity sensors. In all of these background art systems, a photodetector is used to measure light or radiation in terms of an electrical signal that is processed in various ways to produce a useful information output. In a particular example from the background art, a beam of collimated light, which may or may not be coherent, is directed through a transparent cell in which particles suspended in fluid mixtures are made to pass. Photodetectors are then used to detect the relative amount of light that is scattered or blocked by the particles. The signals generated by the photodetector contain information about the concentration of particles, size of particles, and/or presence of particles.
The type of photodetector used depends on the sensitivity requirements of the device. A photo-multiplier tube is the most sensitive (and costly) method that is currently available. A photo-multiplier can detect the presence of a single photon with nanosecond resolution. However, photo-multiplier tubes are very costly to manufacture and are easily damaged. Additionally they have very high voltage requirements and therefore tend to be used in laboratories rather than in commercial applications.
One alternative to using a photo-multiplier tube is to use a photodiode and a transimpedance amplifier. In contrast to a photo-multiplier, photodiodes are inexpensive, rugged, small, and operate at low voltages.
Another background art device that is used to measure aerosol particle size and concentration is called a light scattering photometer or nephelometer. Applications that require particularly sensitive measurements require photo-multiplier-based photometers.
When the sensitivity requirements of the application do not justify the use of a photo-multiplier tube, a photodiode-based device is preferred due to the reduced cost. However, photodiodes are generally not as sensitive as photo-multiplier tubes and are prone to noise problems associated with electrical amplification.
Attempts have been made to enhance the signal output from photodetectors. One such attempt is described in U.S. Pat. No. 7,256,384, the disclosure of which is incorporated herein by reference. However, while this approach manages to enhance the signal output from a photodetector, it still suffers from the same shortcoming as other methods known in the prior art—each measurement output from the photodetector represents an instantaneous measurement of a discrete moment in time. Thus, signal enhancement techniques currently practiced generally average a number of samples taken over a period of time. In this way, current techniques sample the rapidly changing signal output from the photodetector and use the average of multiple samples as a representative output of the photodetector.